1. Field of the Invention
This invention relates to the field of mask defect inspection. More specifically, characterizing defect detection sensitivity on inspection tooling.
2. Background of the Invention
In order to use an inspection tool effectively, its ability to detect defects or sensitivity must quantified. The threshold for detecting defects can vary from tool to tool and pattern to pattern. The standard method for assessing an inspection tool's sensitivity is to select relevant base mask patterns, insert programmed defects of many types and sizes, and then build the mask and inspect it on the inspection tool under test. The tool must be thoroughly characterized against a set of appropriate programmed defects, on mask designs that both challenge the inspection tool and are representative of the product that will be inspected on that tool. Test masks that are currently available consist of simple defect types programmed into simple background patterns, and seldom challenge the abilities of the mask inspection systems and may not be relevant to a specific customer.
It is important to understand the current practice of defect test mask generation. Mask are built with defects programmed into a base design. With the current method of building defect test masks, defects may be sized, for example, from 5 nm to 100 nm in 5 nm increments. The sizing of these defects on the final mask is difficult to predict and may not even resolve. Those that do resolve may have large gaps in size from row to row. Let us assume that an inspection tool sensitivity of 40 nm is needed. If the tool detects a 50 nm defect 100% of the time, but the next smallest defect is 25 nm, and it is only captured 60% of the time, the true capability of the tool is somewhere between 50 and 25 nm. This is not enough granularity to guarantee a 40 nm defect sensitivity.
Defects that resolve on the masks are then measured to determine sizing since there is not a one-to-one correspondence between design and printed defect. Multiple inspections from multiple inspection tools are used to calculate the probability of detection by defect size and type. Purchase specifications and defect sensitivity commitments are then interpolated from those results. Those commitments are based on how well defect types print and measure on a variety of test masks rather than how capable the inspection tool is. This mismatch in commitment versus actual capability is largely based on the quality and size of the defects which resolve on test masks.
In addition, the defect capability must be determined on multiple mask material types. This either multiplies the work or dictates that only a subset of masks will be used to represent the entire population of mask types.
Current test mask practices yields specifications that are tied to one or more physical test masks rather than to actual tool capability. The result is that the commitments are conservative and the test masks themselves become a portion of the specification of a tool. This creates risk for both the tool owner and the inspection tool supplier.